1. Field of the Invention
This invention relates to a process for preparing ethylene-acrylic acid copolymers or salts thereof. The ethylene-acrylic acid copolymer is prepared by first preparing an ethylene-alkyl acrylate copolymer, saponifying such ethylene-alkyl acrylate copolymer in the molten state, in the absence of solvent or water other than by-product alkanol, with alkali metal or alkaline earth metal hydroxide to form alkanol and a salt, or an ionomer, of ethylene-acrylic acid copolymer and, optionally, then acidifying such salt to form the ethylene-acrylic acid copolymer.
2. Description of the Prior Art
Ethylene-acrylic acid copolymers may be made directly by copolymerizing ethylene and acrylic acid and salts, or ionomers, of the resulting ethylene-acrylic acid copolymers can then be prepared from such copolymers. However, where the available polymerization reactors and auxiliary equipment are made of carbon steel, the acrylic acid monomer, as well as the ethylene-acrylic acid copolymer made therefrom, may seriously corrode such reactors and equipment. Further, although no such corrosion problems are encountered when copolymerizing ethylene and an alkyl acrylate in carbon steel reactors, it is still necessary to saponify the ethylene-alkyl acrylate copolymers if one wishes to prepare a salt of the ethylene-acrylic compolymer may be prepared. The prior art saponification processes require the use of relatively large amounts of added solvents or water which must be removed from the desired products.
U.S. Pat. No. 3,249,570 discloses a three step process for partially saponifying ethylene-alkyl acrylate copolymers in which the ethylene-alkyl acrylate copolymers are first saponified in a quaternary solvent/water system, then subjected to a pH adjustment step and then recovered by precipitation from an organic liquid non-solvent.
U.S. Pat. No. 3,970,626 discloses a process for saponifying ethylene-alkyl acrylate copolymers in the presence of an aqueous solution of an excess of alkali metal hydroxide. The reaction is conducted at temperatures of at least 180.degree. C. under autogenous pressure in a pressurized reactor.
U.S. Pat. No. 4,307,211 discloses the preparation of selected ethylene-ethyl acrylate-acrylic acid terpolymers by the hydrolysis of ethylene-ethyl acrylate copolymer with water or steam in an inert atmosphere and at temperatures of 150.degree. to 450.degree. C. in a pressurized reactor. The reaction also apparently involves a chain scission of the terpolymer.
U.S. Pat. No. 3,472,825 discloses the preparation of water soluble salts of ethylene-acrylic acid copolymer by neutralizing the copolymers with alkali metals or alkali metal based compounds.